1. Field of the Invention
The present invention relates to a voltage-controlled oscillator (VCO) that is used in a wireless communication device, and a PLL circuit, an FLL circuit, and a wireless communication device, all of which use the voltage-controlled oscillator.
2. Description of the Background Art
Conventionally, a voltage-controlled oscillator is widely used as a device for generating a local oscillation signal of a wireless communication device. FIG. 12 is a figure showing a configurational example of a conventional voltage-controlled oscillator. The voltage-controlled oscillator shown in FIG. 12 includes: two inductors 11 and 12; two variable capacitors 21 and 22; a switching element 32; two capacitive elements 31 and 33; two oscillation transistors 41 and 42; and a current source 50. A constant power-supply voltage Vdd is applied to a connection point between the inductors 11 and 12 by an LDO (low-dropout regulator). In addition, a control voltage Vt is applied to a connection point between the variable capacitors 21 and 22; and a control signal Fse1 is inputted into the switching element 32. This voltage-controlled oscillator changes the capacitance value of the whole voltage-controlled oscillator by controlling the control voltage Vt and the control signal Fse1. As a result, an oscillation frequency of the local oscillation signal outputted from the voltage-controlled oscillator changes. In FIG. 13, a solid line indicates a change in the oscillation frequency with respect to the control voltage Vt when the control signal Fse1 is LOW; and a dashed line indicates a change in the oscillation frequency with respect to the control voltage Vt when the control signal Fse1 is HIGH.
If a temperature change occurs while the voltage-controlled oscillator is in use, for example, the inductances of the inductors 11 and 12 change in association with the temperature change. As a result, the oscillation frequency of local oscillation signal fluctuates. A conceivable way to suppress such a fluctuation of the oscillation frequency is to sense the temperature of the voltage-controlled oscillator, and, depending on the result of the sensing, change the control voltage Vt within a variable range. Here, the variable range of the control voltage Vt shown in FIG. 13 is a range in which a value of the control voltage Vt can fall within, and a stable oscillation frequency characteristic cannot be obtained beyond this range. Known means for sensing the temperature of the voltage-controlled oscillator are those disclosed in Japanese Laid-Open Patent Publication No. H10-126154 (hereinafter, denoted as patent document 1) and Japanese Laid-Open Patent Publication No. 1109-223929 (hereinafter, denoted as patent document 2). As the means for sensing the temperature of the voltage-controlled oscillator, patent document 1 discloses a temperature compensation circuit that includes a resistance and an NTC thermistor, and patent document 2 discloses an NTC thermistor.
When a temperature change described above occurs, a temperature characteristic of the oscillation frequency will be one as shown in FIG. 14A. In FIG. 14A, a solid line indicates the oscillation frequency characteristic at a normal temperature, a dotted line indicates the oscillation frequency characteristic at a low temperature, and a dashed line indicates the oscillation frequency characteristic at a high temperature. Here, at a low temperature, suppose f0 is selected as the oscillation frequency by configuring the control voltage Vt as VtA which is within the variable range. Next, when the temperature increases under this condition, it is necessary to change the control voltage Vt from VtA to VtB in order to hold the oscillation frequency at f0. However, the oscillation frequency cannot be held at f0 since VtB is a voltage value that is beyond the variable range of the control voltage Vt. Thus, in a case where the oscillation frequency changes due to the temperature, as shown in FIG. 14B, it is necessary to narrow an interval of a frequency band between an Fse1 at LOW and an Fse1 at HIGH, such that VtB will be in the variable range of the control voltage Vt. Therefore, with a method of holding the oscillation frequency by adjusting the control voltage Vt in response to a change of the inductance due to a temperature change, there is a problem where the variable range of the oscillation frequency becomes narrow.